The deposition of organic contaminants in the pulp and paper industry can cause both quality and efficiency problems in pulp and papermaking systems. Some components occur naturally in wood and are released during various pulping and papermaking processes. The term "pitch" can be used to refer to deposits composed of organic constituents which may originate from these natural resins, their salts, as well as coating binders, sizing agents, and defoaming chemicals which may be found in the pulp. In addition, pitch frequently contains inorganic components such as calcium carbonate, talc, clays, titanium, and related materials.
Stickies is a term that has become increasingly used to describe deposits that occur in systems using recycled fiber. These deposits often contain the same material found in "pitch" deposits in addition to adhesives, hot melts, waxes, and inks. All of the aforementioned materials have many common characteristics including: hydrophobicity, deformability, tackiness, low surface energy, and the potential to cause problems with deposition, quality, and efficiency in the process. Diagram I shows the complex relationship between pitch and stickies discussed here.
______________________________________ DIAGRAM I Pitch Stickies ______________________________________ Natural Resins (fatty and resin acids, fatty esters, X X insoluble salts, sterols, etc.) Defoamers (oil, EBS, silicate, silicone oils, X X ethoxylated compounds, etc.) Sizing Agents (Rosin size, ASA, AKD, X X hydrolysis products, insoluble salts, etc.) Coating Binders (PVAC, SBR) X X Waxes X Inks X Hot Melts (EVA, PVAC, etc.) X Contact Adhesives (SBR, vinyl acrylates, X polyisoprene, etc.) ______________________________________
The deposition of organic contaminants can be determental to the efficiency of a pulp mill causing both reduced quality and reduced operating efficiency. Organic contaminants can deposit on process equipment in papermaking systems resulting in operational difficulties in the systems. The deposition of organic contaminants on consistency regulators and other instrument probes can render these components useless. Deposits on screens can reduce throughput and upset operation of the system. This deposition can occur not only on metal surfaces in the system, but also on plastic and synthetic surfaces such as machine wires, felts, foils, Uhle boxes and headbox components.
Historically, the subsets of the organic deposit problems, "pitch" and "stickies" have manifested themselves separately, differently have been treated distinctly and separately. From a physical standpoint, "pitch" deposits have usually formed from microscopic particles of adhesive material (natural or man-made) in the stock which accumulate on papermaking or pulping equipment. These deposits can readily be found on stock chest walls, paper machine foils, Uhle boxes, paper machine wires, wet press felts, dryer felts, dryer cans, and calendar stacks. The difficulties related to these deposits included direct interference with the efficiency of the contaminated surface, therefore, reduced production, as well as holes, dirt, and other sheet defects that reduce the quality and usefulness of the paper for operations that follow like coating, converting or printing.
From a physical standpoint, "stickies" have usually been particles of visible or nearly visible size in the stock which originate from the recycled fiber. These deposits tend to accumulate on many of the same surfaces that "pitch" can be found on and causes many of the same difficulties that "pitch" can cause. The most severe "stickies" related deposits however tend to be found on paper machine wires, wet felts, dryer felts and dryer cans.
Methods of preventing the build-up of deposits on the pulp and papermill equipment and surfaces are of great importance to the industry. The paper machines could be shut down for cleaning, but ceasing operation for cleaning is undesirable because of the consequential loss of productivity, poor quality while partially contaminated and "dirt" which occurs when deposits break off and become incorporated in the sheet. Preventing deposition is thus greatly preferred where it can be effectively practiced.
In the past stickles deposits and pitch deposits have typically manifested themselves in different systems. This was true because mills usually used only virgin fiber or only recycled fiber. Often very different treatment chemicals and strategies were used to control these separate problems.
Current trends are for increased mandatory use of recycled fiber in all systems. This is resulting in a co-occurrence of stickles and pitch problems in a given mill. It is desirable to find treatment chemicals and strategies which will be highly effective at eliminating both of these problems without having to feed two or more separate chemicals. The materials of this invention have clearly shown their ability to achieve this goal.
Pitch control agents of commerce have historically included surfactants, which when added to the system, can stabilize the dispersion of the pitch in the furnish and white water. Stabilization can help prevent the pitch from precipitating out on wires and felts.
Mineral additives such as talc have also found use and can reduce the tacky nature of pitch by adsorbing finely dispersed pitch particles on their surfaces. This will reduce the degree to which the particles coagulate or agglomerate.
Polyphosphates have been used to try to maintain the pitch in a finely dispersed state. Alum has also been widely used to reduce deposition of pitch and related problems.
Both chemical and non-chemical approaches to stickles control are employed by papermakers. Non-chemical approaches include furnish selection, screening and cleaning, and thermal/mechanical dispersion units.
Chemical treatment techniques for stickles control include dispersion, detackification, wire passivation and cationic fixation. Chemicals used included talc, polymers, dispersants and surfactants.